In Vitro Fertilization (IVF) often involves transferring multiple embryos into the uterus to improve the chances of a successful pregnancy. This practice, known as Double Embryo Transfer (DET), is an established method to maximize the odds of implantation. However, transferring two embryos frequently results in a singleton pregnancy, meaning only one of the transferred embryos successfully develops. This common outcome shifts the focus of care to supporting a single developing fetus.
Defining the Implantation Scenario
Transferring two embryos aims to increase the likelihood of at least one implantation, but the rate of both implanting is lower than many assume. A significant percentage of pregnancies following DET result in a single live birth. This occurs when one embryo successfully attaches to the uterine wall and begins to develop, while the second embryo either fails to implant entirely or ceases development very early.
This scenario is typically confirmed during the first transvaginal ultrasound, usually scheduled around six to eight weeks of gestation. At this early stage, the healthcare provider identifies the number of gestational sacs and embryos with a detectable heartbeat. Observing only one heartbeat confirms the establishment of a singleton pregnancy. If the second embryo had implanted but failed, this leads to the specific medical phenomenon known as Vanishing Twin Syndrome.
Understanding Vanishing Twin Syndrome
The term Vanishing Twin Syndrome (VTS) occurs when an early ultrasound detects two or more gestational sacs, but a subsequent scan shows only one developing fetus. This spontaneous reduction to a singleton pregnancy is common, estimated to affect 20% to 30% of pregnancies achieved through assisted reproductive techniques. VTS is a form of early miscarriage where one embryo is lost while the other continues to thrive.
The biological mechanism involves the reabsorption of the non-viable fetal tissue by the mother’s body, the placenta, or the surviving twin. In most cases, the loss is attributed to developmental issues or chromosomal abnormalities in the non-surviving embryo, preventing continued growth. Since the loss typically occurs in the first trimester (before 12 weeks), the process is often asymptomatic, though some mothers may experience light bleeding or mild cramping.
The reabsorption of tissue usually requires no medical intervention and does not harm the continuing pregnancy. Only in rare instances, when the loss occurs later, may the deceased fetus be compressed into a flattened state known as fetus papyraceus. For first-trimester cases, the body naturally processes the tissue, making the former twin appear to have “vanished.”
Monitoring and Clinical Management
Once singleton status is confirmed, clinical management shifts to supporting a standard IVF singleton pregnancy. Initial monitoring involves a series of early transvaginal ultrasounds, typically performed around six, eight, and twelve weeks of gestation, to ensure proper development. This close monitoring allows the healthcare team to observe the surviving fetus and gestational environment.
Management consists of standard prenatal care and reassurance, without the need for special interventions related to the lost embryo. The pregnancy may still be considered higher risk simply because it was conceived through IVF and required a double embryo transfer. The focus remains on the health and growth of the surviving singleton, using the established prenatal testing schedule.
Specific monitoring may include checking maternal serum markers for Down syndrome screening, as the presence of a non-viable twin can potentially alter these blood test results. Healthcare providers continue to monitor the patient for symptoms like bleeding or cramping, which can occur with VTS, though they are also common in early pregnancy.
Prognosis and Long-Term Outcomes
The prognosis for the surviving singleton is generally excellent, especially when the loss of the co-twin occurs early in the first trimester. Converting to a singleton significantly reduces the risks associated with twin pregnancies, such as high rates of preterm birth and low birth weight.
Studies show that singletons born after a VTS diagnosis may have a slightly higher rate of adverse outcomes compared to singletons conceived without co-twin loss. Specifically, there is a small, statistically relevant increased risk for outcomes like preterm delivery and low birth weight. This subtle difference is linked to factors such as the underlying reasons for the initial twin loss or potential issues with placental development.
The long-term health of the mother is typically not negatively affected by the early loss of the co-twin. Care focuses on standard high-quality obstetric management for the remainder of the pregnancy, acknowledging the slightly elevated baseline risks associated with assisted reproductive technology.